The goal of this work is to establish the molecular mechanism by which the visual pigment, rhodopsin, modulates the internal level of c-GMP in retinal photoreceptor cells. This process, which bears a striking resemblence to the regulation of the cyclase activity by hormone receptors, is currently considered to be an important step in visual transduction. Three proteins are involved: Rhodopsin, a GTP binding protein (GTPase) and a phosphodiesterase (PDE) specific to c-GMP. The activation of the PDE seems to be preceded by light induced changes in the interactions between rhodopsin and the GTPase. The proposed study is aimed to explore the later interactions and their regulation by light. We shall use model membrane vesicles consisting of purified rhodopsin reassembled with phospholipid, and supplement them with purified enzyme. In such reconstitution systems spectdroscopic and chemical techniques can be employed, and the results can be interpreted with less ambiguity. We shall use fluorescence energy transfer, chemical crosslinking and enzyme activation to follow protein-protein interactions. The study is designed to assess steady state and kinetic parameters of the interactions between rhodopsin and the GTPase, as well as their sensitivity to rhodopsin photolysis. The time scale of the light induced events and the associated amplification gain will be investigated. To assess a possible role for themembrane fluidity, the effect of the membrane lipid composition on the process will be studied.